1. Field of the Invention
The invention relates to a focus servo control method in an optical disc device, and, more particularly, to a method for performing focus servo control while compensating for variations present on an optical disc.
2. Background of the Related Art
Optical discs have been developed recently which allow data to be recorded on a data surface of the disc, and a label with a desired design to be printed on the opposite, or non-data, surface of the disk. The surface of the disc on which a label is printed is referred to as the label surface of the disc. Labels are typically transferred onto the label surface of the disc using a laser etching technology commonly referred to as LightScribe, in which a laser burns or etches an image onto a specially prepared, non-data side of an optical disc.
A recording layer and a reflective layer are formed on the data surface of a recordable optical disc. When an optical pickup of an optical disc device radiates a laser beam onto the data surface, the laser beam is reflected by the reflective layer and is then incident on a photo-detector (PD) of the optical pickup. Focus and tracking error signals can be produced using signals output from divided cells of the photo-detector.
When reproducing data from or recording data to a disc, the optical disc device performs a focus servo operation which moves an objective lens of the optical pickup up and down, and a tracking servo operation which moves the objective lens in a radial direction so as to minimize the levels of focus and tracking error signals generated based on the signal reflected from the data surface of the disc. Servo control on the data surface of the disc based on the feedback error signals is referred to as feedback servo control.
However, the label surface of the disc preferably has no reflective layer, and is preferably rough compared to the data surface. Thus, the focus error signal required to perform feedback focus servo control cannot be generated, and feedback focus servo control cannot be performed for the label surface of this type of disc. Thus, there is no choice but to perform feed-forward focus servo control.
However, feed-forward focus servo control cannot compensate for any surface vibration components or unevenness in the optical disc, particularly on the label surface of the optical disc. This inability to compensate causes a fatal error in the label printing operation, and a failure to properly print a label with a desired image or design on the disc
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.